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Review: Deep Impact

The Astronomy of Deep ImpactThe producers of DI had an ambitious plan: actually collect a
team of scientists and ask them about the scientific plausibility of
the movie. This has been done before (as I mentioned
in my page about the movie "Asteroid"',
NBC claims to have used consultants; I assume they simply ignored
everything the scientists said), but this time, it appears that the
producers listened. At least for the most part; there were plenty
of small errors (many of which I will happily attribute to "artistic
license", see below), and one mighty big one, but a lot of the movie was
accurate. I get the impression that the producers of DI wanted to
make an accurate movie, and used the consultant's info as much as they
could, but I also get the impression they had a certain plot in mind
and would not change parts of it no matter how inaccurate they were.
The first and last few minutes of the movie imply this very strongly,
and I'll describe that when I describe the scenes in question.

Now, on to the list of inaccuracies. I will try to indicate whether I think
the bad astronomy was plot driven (in other words, kept because the plot
demanded it) or simply a goof. Also, to be fair, if the movie got it right
(and it very often did!) I will say so. Incidentally, the images of the
movie shown below (and on the previous page are
from the official
Deep Impact Site. [NOTE (5/3/99): The site I originally got
the images from has been removed, but the site linked looks exactly like
it. I assume they simply changed domain names.] Also, at the bottom of
this page is a list of links to other sites about comet
impacts that you are welcome to browse. Take this chance to learn
more about comets and their effects!

Bad:
In the opening scene (why waste time? ;-) we see a
group of young amateur astronomers stargazing.
As we pan across the group, we see
them studying maps with flashlights.

Good:
The problem here is small: using a flashlight outside
at night ruins your dark vision. The eye takes quite a
bit of time to get adapted to the darkness; usually twenty minutes to get
fully adjusted (less if you are in a light polluted area). Using a white
flashlight destroys the very reason you're outside! However, the eye's
dark adaption is not ruined by red light, so astronomers use red flashlights
(usually modified by the high tech method of taping red cellophane over the
business end of the flashlight). This may have been plot-driven; we'd want to
be able to see the actor's faces.

Bad:
A moment later in this same scene, we come upon Our Hero,
Leo Beiderman (Elijah Wood). He is observing the stars
Mizar and Alcor
through his telescope, situated
right next to his (soon-to-be) girlfriend. Through the 'scope, we can
clearly see Mizar, Alcor, and a third object at about the same brightness.
This third object turns out to be (ominous chord please) A Comet.

Good:
Mizar and Alcor are real stars. Mizar is the relatively
bright star in the kink in the handle of the Big Dipper in Ursa Major.
At about second magnitude, it is easy to find in even light polluted
skies. Alcor is a dimmer companion to Mizar in the sky, and can be a bit
tough to spot (it was once used as an eye test by ancient Arabians). The
problem is, The Comet appears as bright as Mizar. If it were that bright,
literally thousands of people would have spotted it much earlier.
Comets do not move very quickly in the sky, so
it would have taken quite a bit of time to get that bright. Someone
would have spotted it months before. This is clearly plot-driven; Leo
is the one to spot The Comet, and it perhaps would seem out of place
for him to have a large enough 'scope to spot The Comet before the
thousands of other people also looking for comets. I'll note here that
many comets are spotted using relatively small 'scopes, but that is
the exception. Most are discovered by very dedicated people with
very good equipment.

Incidentally, there is a hidden piece of Bad Astronomy in this scene as well:
the government would have no chance of hiding the existence of this comet
for a year. Most comets are discovered when they are very faint, literally
thousands of times fainter than Wolf-Beiderman when Leo found it. Long
before the events in the film take place, probably even before the government
itself would be notified, the comet would have been discovered, an orbit
calculated, and people panicked. The whole premise of the first half of
the movie is thus based on flawed logic. My thanks to Bad Reader David Takemoto-Weerts for reminding me of this;
I had thought of it while watching the movie but forgot about it when I wrote
up this page originally!]

The movie did get a couple of points correct here though: most
comets are indeed discovered by amateurs, and are named after the
discoverers. That is why The Comet was named Wolf-Beiderman in the
movie.

Bad:
Leo reports his discovery to the local professional
astronomer, who programs his software to plot a course for The Comet.
It displays a three-dimensional graphic which shows The Comet headed
right for Earth. In his panic to report the discovery, he wrecks his car.
Kaboom!

Good:
Leo should have reported the discovery to
Brian Marsden at the
International Astronomical Union's Minor Planet Center. We can forgive him
this breach of professional etiquette since Leo wanted to report to his club's
sponsor first. The real problem is that it takes several observations
over many nights to get a good orbital calculation for comets, especially one
that's a year away from impact. It's like looking at a snapshot of a baseball
and being asked when it will hit the ground. What direction is it heading?
How fast is it moving? One observation doesn't really tell you
anything; you need to see it move. The 3D graphic was a bit over the top too.
While not really impossible, it's more of a Hollywood silliness.
It's there simply to drive the point home that we're in for a bit of trouble.
And I have a plot review here too: killing the pro was just plain silly,
and was telegraphed for way too long. Was anyone really surprised when
his car crashed (and exploded??? Puhleease!)?

Bad:
The professional astronomer, when we first see him, is in an observatory.
He is sitting in the same room as the telescope, with lights, computers,
etc. running.

Bad:
We see the group of young astronauts sitting at a bar
grousing about having the older more experienced astronaut on their team.

Good:
This is my own opinion here, but what I have seen of
astronauts is a lot of respect for the ones that are veterans of many flights.
I got the impression that Robert Duvall's character was based loosely on
Story Musgrave,
a Shuttle astronaut of almost unbelievable record. He
holds seven degrees (including a doctorate in medicine),
and has been on six Shuttle flights, including
the first Hubble Space Telescope servicing mission. He's a legend at
NASA, and I have a hard time picturing a group of young astronauts
sitting around saying anything about him except that they wish they could
do half as well. I didn't like that scene in DI, and it was
obviously only placed in there for character conflict.

Bad:
The President says we don't have any spacecraft that can catch
up to The Comet, so they need an Orion type engine.

Good:
This scene is mostly good, and the Badness is an omission.
We really don't have any spacecraft that could rendezvous with an object
moving at 40 or more kilometers per second (note that there were probes sent
to Comet Halley in 1985, but they were fly-by's, not landers). The Orion,
in turn, is a real idea: you build a ship on top of a massive shield, and
(hold on to your seats) blow up nuclear bombs under the shield. The explosion
propels the ship (imagine huge shock absorbers). This idea was shelved
a long time ago because a treaty was signed by many nations not to blow up
nuclear weapons in space.
Still, the concept is actually real. My problem
with the movie is that it is never explained; it would only take a few
lines to say what it was, and they never did. My point is that Orion
is real, and if they dropped a line or two about its history they
might have inspired some kids to look it up. A great opportunity to teach
was lost (although maybe in my own small way I can try to revive it).

Bad:
We send astronauts to land on The Comet to blow it up.

Good:
This is a difficult subject. No one is really quite sure just what the
best way would be to prevent a comet from impacting the Earth. One
of the most important factors is time. If you have a long lead time,
like say twenty years, then you would take a different approach then if
you had a month or a year. In general a comet is a big lump of ice
orbiting the Sun. If you give it a little nudge, it might only move
a little bit, but that movement never stops; the comet gets more and more
"off-course" as time goes on. Imagine it's originally aimed at Earth, and
we give it a small nudge. If enough time can elapse between our nudge and the
time it gets near the Earth, it may have moved enough from the original
path to completely miss the Earth! So if you have enough time, even a
small nudge can save you. The closer it is, the less time you have, and
the larger a nudge you have to give it.

A back-of-the-envelope calculation shows (if you have help from
Bad Reader Mati Meron, who is in no way responsible for any errors
I might be making here!) that you need about 150 or so megatons
to push the comet hard enough to have it miss the Earth entirely,
given a year's warning (half that if it's two years)
with the mass I find for Wolf-Beiderman.
Unless I have made
some large error in my math, it seems to me we need never send astronauts
to The Comet at all; just set up a series of bombs to blow up near
it and let Newton do the driving.

Blowing up The Comet is a different story.
To start with, blowing it
up might be a very bad thing to do.
But if you really want to blow it up,
you could. I have heard people suggest
simply throwing rocks, literally, at an incoming comet, but that may
not pack quite enough punch; a one ton rock slamming into a comet
would only provide about one-ten-thousandth the energy of a one
megaton bomb.
The idea of getting bombs inside the
comet is a good one; if you can disrupt the comet enough, perhaps
jet effects will also drive it off course (or, given enough time,
the expanding debris will get big enough that most of it misses the Earth).
I have my doubts about actually
being able to disrupt the comet, but I don't know enough about explosive
yields and comet tensile strength to even guess if we can actually blow
one up (there are some indications, however,
that the comet that
slammed into Jupiter in 1994 was only very loosely held together;
it was likened to a pile of gravel). Anyway, there are ways to get bombs inside a comet without needing
people there; for example, drill heads attached to the missiles, or very hard
tipped bombs that would actually drive themselves into the surface upon
impact (some probes to Mars are actually using something similar to this
to sample the subsurface region). If we wanted to drill bombs under the
surface, then I would be happier if we had some real live astronauts
up there, capable of thinking their way out of a problem. They could be
used as a sort of backup in case the machines themselves don't work.

So in DI my problem is manyfold: we don't want to blow up The Comet,
we want to push it out of the way. We had the time (two years, as stated
in the movie). And if we did want to blow it up, sending astronauts
is a good idea as a backup in case of automation failure, not as a
primary defense.

Bad:
The astronauts are worried while they are at The Comet because
if they dally too long, the Sun will rise over their landing spot and
cause The Comet to vent. Sure enough, they are delayed, and as the
sunlight hits the nearby surface of The Comet, vents immediately open up.

Good:
Comets do vent like that. Comets are made up of frozen gasses,
and warm sunlight can vaporize the gas. If the comet rotates, part of it
is in sunlight while part is in shadow (just like here on Earth). Usually,
the more volatile gasses bleed away from the surface while the comet is
far from the Sun, since they have a lower melting point, but deep down they
are insulated by the ice above. A deep fissure or crack in the surface may
expose some of those gasses to the warm light, creating a vent. As the gas
evaporates, it can actually create jets that shoot out of the surface.
This happened quite dramatically with the last big comet, Hale-Bopp.(That link will also show you a very cool picture of Hale-Bopp's
jet as well, check it out!)
The problem is, the sunlight would have to get down the fissure to warm
the gas, which would only happen when the sun was relatively high in the sky.
Those jets in the movie started as soon as sunlight hit the surface, which
has had lots of time to bleed off the nasty gas. In reality, the astronauts
would have had more time. I realize this is a minor point, but comet
vents are pretty cool and I wanted the chance to talk about them. ;-)

Bad:
One of the away team astronauts is blinded by the sunlight.

Good:
I suspect this may not be totally inaccurate; I
am not sure how close The Comet was to the Sun
when this happened. I would expect NASA to
give them sun visors, however, or build in a UV block to the visor.
They were on the dark side of The Comet, after all, so they would not have had
their sun visors down, and he was admittedly too panicked to think of
snapping it down. Still, a moment's exposure should not have so badly burned
his face. That was added for drama.

Bad:
While Jenny Lerner (Tea Leoni) is memorializing the lost astronaut,
an MSNBC graphic shows his face next to an image of The Comet.
I was luckily able to find the very picture in question (shown
to the left). You can see the comet in the picture above the
"EA" in the word "EARTH".

Good:
The comet image was actually a
Hubble picture of Comet Hyakutake, the
bright comet that graced our skies shortly before Hale-Bopp landed on the
scene. This isn't Bad, but I just thought I should point it out.
The original HST image is shown to the right.

The Comet was also moving much faster than the speed of sound as well.
This means it would generate a tremendous shock wave, the force of
which would have smashed flat everyone watching within hundreds of
kilometers. They would never have heard The Comet; they would have been
squished into jelly before the sound could reach them. Yuck.
In the image to the right, you can see the shock wave as a circular
front with the impact in the middle. The impact graphics were very
good.

Bad:
Moments after impact, a wall of water rises up. The water near the beach
recedes drastically, supposedly because it is being drawn up into the wave.
We hear the huge roar of the water as it approaches landfall.

Good: As the President said in his speech, the water displaced
from the impact would be moving faster than sound. What that means is that
water far away from the impact would have no warning when the wave hits it;
therefore the water would not have receded. As far as the water
near the beach is concerned, nothing unusual is happening, until the
wave is literally on top of the beach. It was a cool graphic, but not
realistic.
[Note: I have been informed after posting this that the speed of
the wave is supersonic in the air, but not in water. Sound travels
faster in water, so it is possible that the wave has slowed enough
that water on the beach is aware of the tsunami, before the
tsunami actually hits. I am still not clear on why the beach water
would recede in this case, but I have also been told that
in earthquake induced tsunamis the water in front of the wave
actually does recede. Hmmm.]

Also, the sound from the water is moving only marginally faster than the
water itself. The water initially starts out moving faster than sound,
but slows as it approaches landfall (the movie got this part correct).
The sound from the wave would be in front of the wave itself by only a few
seconds. You would see the wave approaching silently for a few seconds
before hearing it (discounting the fact that sound travels faster
through solid ground, so you might feel it through the ground
beneath your feet).
[Note again: The wave does slow as it moves towards land. It is
possible that is has slowed enough, especially traveling the couple of
hundred kilometers from the ocean to DC, that it lacked a lot of its initial
punch by the time it hit the Capitol.]

One last note: comet impacts and the like are old hat to science fiction
fans. Deep Impact was in some ways similar to the classic novel "Lucifer's
Hammer", written by Larry Niven and Jerry Pournelle. This book
sparks a bit of controversy among fans about its quality, but I
liked it quite a bit. The descriptions of the comet impact are
wonderful (if terrifying) and the aftermath is in my opinion pretty
accurate. The characters are also fun and interesting. I recommend it!
A good library or bookstore will carry it. A web search at
Yahoo! will also yield some web sites with
reviews of the book.